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                                <section class="normal markdown-section">
                                
                                <h1 id="&#x6570;&#x7EC4;">&#x6570;&#x7EC4;</h1>
<h2 id="&#x5B9A;&#x4E49;">&#x5B9A;&#x4E49;</h2>
<p>&#x6570;&#x7EC4;&#xFF08;array&#xFF09;&#x662F;&#x6309;&#x6B21;&#x5E8F;&#x6392;&#x5217;&#x7684;&#x4E00;&#x7EC4;&#x503C;&#x3002;&#x6BCF;&#x4E2A;&#x503C;&#x7684;&#x4F4D;&#x7F6E;&#x90FD;&#x6709;&#x7F16;&#x53F7;&#xFF08;&#x4ECE;0&#x5F00;&#x59CB;&#xFF09;&#xFF0C;&#x6574;&#x4E2A;&#x6570;&#x7EC4;&#x7528;&#x65B9;&#x62EC;&#x53F7;&#x8868;&#x793A;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [<span class="hljs-string">&apos;a&apos;</span>, <span class="hljs-string">&apos;b&apos;</span>, <span class="hljs-string">&apos;c&apos;</span>];
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x4E2D;&#x7684;<code>a</code>&#x3001;<code>b</code>&#x3001;<code>c</code>&#x5C31;&#x6784;&#x6210;&#x4E00;&#x4E2A;&#x6570;&#x7EC4;&#xFF0C;&#x4E24;&#x7AEF;&#x7684;&#x65B9;&#x62EC;&#x53F7;&#x662F;&#x6570;&#x7EC4;&#x7684;&#x6807;&#x5FD7;&#x3002;<code>a</code>&#x662F;0&#x53F7;&#x4F4D;&#x7F6E;&#xFF0C;<code>b</code>&#x662F;1&#x53F7;&#x4F4D;&#x7F6E;&#xFF0C;<code>c</code>&#x662F;2&#x53F7;&#x4F4D;&#x7F6E;&#x3002;</p>
<p>&#x9664;&#x4E86;&#x5728;&#x5B9A;&#x4E49;&#x65F6;&#x8D4B;&#x503C;&#xFF0C;&#x6570;&#x7EC4;&#x4E5F;&#x53EF;&#x4EE5;&#x5148;&#x5B9A;&#x4E49;&#x540E;&#x8D4B;&#x503C;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [];

arr[<span class="hljs-number">0</span>] = <span class="hljs-string">&apos;a&apos;</span>;
arr[<span class="hljs-number">1</span>] = <span class="hljs-string">&apos;b&apos;</span>;
arr[<span class="hljs-number">2</span>] = <span class="hljs-string">&apos;c&apos;</span>;
</code></pre>
<p>&#x4EFB;&#x4F55;&#x7C7B;&#x578B;&#x7684;&#x6570;&#x636E;&#xFF0C;&#x90FD;&#x53EF;&#x4EE5;&#x653E;&#x5165;&#x6570;&#x7EC4;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [
  {a: <span class="hljs-number">1</span>},
  [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>],
  <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) </span>{<span class="hljs-keyword">return</span> <span class="hljs-literal">true</span>;}
];

arr[<span class="hljs-number">0</span>] <span class="hljs-comment">// Object {a: 1}</span>
arr[<span class="hljs-number">1</span>] <span class="hljs-comment">// [1, 2, 3]</span>
arr[<span class="hljs-number">2</span>] <span class="hljs-comment">// function (){return true;}</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x6570;&#x7EC4;<code>arr</code>&#x7684;3&#x4E2A;&#x6210;&#x5458;&#x4F9D;&#x6B21;&#x662F;&#x5BF9;&#x8C61;&#x3001;&#x6570;&#x7EC4;&#x3001;&#x51FD;&#x6570;&#x3002;</p>
<p>&#x5982;&#x679C;&#x6570;&#x7EC4;&#x7684;&#x5143;&#x7D20;&#x8FD8;&#x662F;&#x6570;&#x7EC4;&#xFF0C;&#x5C31;&#x5F62;&#x6210;&#x4E86;&#x591A;&#x7EF4;&#x6570;&#x7EC4;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [[<span class="hljs-number">1</span>, <span class="hljs-number">2</span>], [<span class="hljs-number">3</span>, <span class="hljs-number">4</span>]];
a[<span class="hljs-number">0</span>][<span class="hljs-number">1</span>] <span class="hljs-comment">// 2</span>
a[<span class="hljs-number">1</span>][<span class="hljs-number">1</span>] <span class="hljs-comment">// 4</span>
</code></pre>
<h2 id="&#x6570;&#x7EC4;&#x7684;&#x672C;&#x8D28;">&#x6570;&#x7EC4;&#x7684;&#x672C;&#x8D28;</h2>
<p>&#x672C;&#x8D28;&#x4E0A;&#xFF0C;&#x6570;&#x7EC4;&#x5C5E;&#x4E8E;&#x4E00;&#x79CD;&#x7279;&#x6B8A;&#x7684;&#x5BF9;&#x8C61;&#x3002;<code>typeof</code>&#x8FD0;&#x7B97;&#x7B26;&#x4F1A;&#x8FD4;&#x56DE;&#x6570;&#x7EC4;&#x7684;&#x7C7B;&#x578B;&#x662F;<code>object</code>&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">typeof</span> [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>] <span class="hljs-comment">// &quot;object&quot;</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x8868;&#x660E;&#xFF0C;<code>typeof</code>&#x8FD0;&#x7B97;&#x7B26;&#x8BA4;&#x4E3A;&#x6570;&#x7EC4;&#x7684;&#x7C7B;&#x578B;&#x5C31;&#x662F;&#x5BF9;&#x8C61;&#x3002;</p>
<p>&#x6570;&#x7EC4;&#x7684;&#x7279;&#x6B8A;&#x6027;&#x4F53;&#x73B0;&#x5728;&#xFF0C;&#x5B83;&#x7684;&#x952E;&#x540D;&#x662F;&#x6309;&#x6B21;&#x5E8F;&#x6392;&#x5217;&#x7684;&#x4E00;&#x7EC4;&#x6574;&#x6570;&#xFF08;0&#xFF0C;1&#xFF0C;2...&#xFF09;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [<span class="hljs-string">&apos;a&apos;</span>, <span class="hljs-string">&apos;b&apos;</span>, <span class="hljs-string">&apos;c&apos;</span>];

<span class="hljs-built_in">Object</span>.keys(arr)
<span class="hljs-comment">// [&quot;0&quot;, &quot;1&quot;, &quot;2&quot;]</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x4E2D;&#xFF0C;<code>Object.keys</code>&#x65B9;&#x6CD5;&#x8FD4;&#x56DE;&#x6570;&#x7EC4;&#x7684;&#x6240;&#x6709;&#x952E;&#x540D;&#x3002;&#x53EF;&#x4EE5;&#x770B;&#x5230;&#x6570;&#x7EC4;&#x7684;&#x952E;&#x540D;&#x5C31;&#x662F;&#x6574;&#x6570;0&#x3001;1&#x3001;2&#x3002;</p>
<p>&#x7531;&#x4E8E;&#x6570;&#x7EC4;&#x6210;&#x5458;&#x7684;&#x952E;&#x540D;&#x662F;&#x56FA;&#x5B9A;&#x7684;&#xFF08;&#x9ED8;&#x8BA4;&#x603B;&#x662F;0&#x3001;1&#x3001;2...&#xFF09;&#xFF0C;&#x56E0;&#x6B64;&#x6570;&#x7EC4;&#x4E0D;&#x7528;&#x4E3A;&#x6BCF;&#x4E2A;&#x5143;&#x7D20;&#x6307;&#x5B9A;&#x952E;&#x540D;&#xFF0C;&#x800C;&#x5BF9;&#x8C61;&#x7684;&#x6BCF;&#x4E2A;&#x6210;&#x5458;&#x90FD;&#x5FC5;&#x987B;&#x6307;&#x5B9A;&#x952E;&#x540D;&#x3002;JavaScript &#x8BED;&#x8A00;&#x89C4;&#x5B9A;&#xFF0C;&#x5BF9;&#x8C61;&#x7684;&#x952E;&#x540D;&#x4E00;&#x5F8B;&#x4E3A;&#x5B57;&#x7B26;&#x4E32;&#xFF0C;&#x6240;&#x4EE5;&#xFF0C;&#x6570;&#x7EC4;&#x7684;&#x952E;&#x540D;&#x5176;&#x5B9E;&#x4E5F;&#x662F;&#x5B57;&#x7B26;&#x4E32;&#x3002;&#x4E4B;&#x6240;&#x4EE5;&#x53EF;&#x4EE5;&#x7528;&#x6570;&#x503C;&#x8BFB;&#x53D6;&#xFF0C;&#x662F;&#x56E0;&#x4E3A;&#x975E;&#x5B57;&#x7B26;&#x4E32;&#x7684;&#x952E;&#x540D;&#x4F1A;&#x88AB;&#x8F6C;&#x4E3A;&#x5B57;&#x7B26;&#x4E32;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [<span class="hljs-string">&apos;a&apos;</span>, <span class="hljs-string">&apos;b&apos;</span>, <span class="hljs-string">&apos;c&apos;</span>];

arr[<span class="hljs-string">&apos;0&apos;</span>] <span class="hljs-comment">// &apos;a&apos;</span>
arr[<span class="hljs-number">0</span>] <span class="hljs-comment">// &apos;a&apos;</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x5206;&#x522B;&#x7528;&#x6570;&#x503C;&#x548C;&#x5B57;&#x7B26;&#x4E32;&#x4F5C;&#x4E3A;&#x952E;&#x540D;&#xFF0C;&#x7ED3;&#x679C;&#x90FD;&#x80FD;&#x8BFB;&#x53D6;&#x6570;&#x7EC4;&#x3002;&#x539F;&#x56E0;&#x662F;&#x6570;&#x503C;&#x952E;&#x540D;&#x88AB;&#x81EA;&#x52A8;&#x8F6C;&#x4E3A;&#x4E86;&#x5B57;&#x7B26;&#x4E32;&#x3002;</p>
<p>&#x6CE8;&#x610F;&#xFF0C;&#x8FD9;&#x70B9;&#x5728;&#x8D4B;&#x503C;&#x65F6;&#x4E5F;&#x6210;&#x7ACB;&#x3002;&#x4E00;&#x4E2A;&#x503C;&#x603B;&#x662F;&#x5148;&#x8F6C;&#x6210;&#x5B57;&#x7B26;&#x4E32;&#xFF0C;&#x518D;&#x4F5C;&#x4E3A;&#x952E;&#x540D;&#x8FDB;&#x884C;&#x8D4B;&#x503C;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [];

a[<span class="hljs-number">1.00</span>] = <span class="hljs-number">6</span>;
a[<span class="hljs-number">1</span>] <span class="hljs-comment">// 6</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x4E2D;&#xFF0C;&#x7531;&#x4E8E;<code>1.00</code>&#x8F6C;&#x6210;&#x5B57;&#x7B26;&#x4E32;&#x662F;<code>1</code>&#xFF0C;&#x6240;&#x4EE5;&#x901A;&#x8FC7;&#x6570;&#x5B57;&#x952E;<code>1</code>&#x53EF;&#x4EE5;&#x8BFB;&#x53D6;&#x503C;&#x3002;</p>
<p>&#x4E0A;&#x4E00;&#x7AE0;&#x8BF4;&#x8FC7;&#xFF0C;&#x5BF9;&#x8C61;&#x6709;&#x4E24;&#x79CD;&#x8BFB;&#x53D6;&#x6210;&#x5458;&#x7684;&#x65B9;&#x6CD5;&#xFF1A;&#x70B9;&#x7ED3;&#x6784;&#xFF08;<code>object.key</code>&#xFF09;&#x548C;&#x65B9;&#x62EC;&#x53F7;&#x7ED3;&#x6784;&#xFF08;<code>object[key]</code>&#xFF09;&#x3002;&#x4F46;&#x662F;&#xFF0C;&#x5BF9;&#x4E8E;&#x6570;&#x503C;&#x7684;&#x952E;&#x540D;&#xFF0C;&#x4E0D;&#x80FD;&#x4F7F;&#x7528;&#x70B9;&#x7ED3;&#x6784;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>];
arr<span class="hljs-number">.0</span> <span class="hljs-comment">// SyntaxError</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x4E2D;&#xFF0C;<code>arr.0</code>&#x7684;&#x5199;&#x6CD5;&#x4E0D;&#x5408;&#x6CD5;&#xFF0C;&#x56E0;&#x4E3A;&#x5355;&#x72EC;&#x7684;&#x6570;&#x503C;&#x4E0D;&#x80FD;&#x4F5C;&#x4E3A;&#x6807;&#x8BC6;&#x7B26;&#xFF08;identifier&#xFF09;&#x3002;&#x6240;&#x4EE5;&#xFF0C;&#x6570;&#x7EC4;&#x6210;&#x5458;&#x53EA;&#x80FD;&#x7528;&#x65B9;&#x62EC;&#x53F7;<code>arr[0]</code>&#x8868;&#x793A;&#xFF08;&#x65B9;&#x62EC;&#x53F7;&#x662F;&#x8FD0;&#x7B97;&#x7B26;&#xFF0C;&#x53EF;&#x4EE5;&#x63A5;&#x53D7;&#x6570;&#x503C;&#xFF09;&#x3002;</p>
<h2 id="length-&#x5C5E;&#x6027;">length &#x5C5E;&#x6027;</h2>
<p>&#x6570;&#x7EC4;&#x7684;<code>length</code>&#x5C5E;&#x6027;&#xFF0C;&#x8FD4;&#x56DE;&#x6570;&#x7EC4;&#x7684;&#x6210;&#x5458;&#x6570;&#x91CF;&#x3002;</p>
<pre><code class="lang-javascript">[<span class="hljs-string">&apos;a&apos;</span>, <span class="hljs-string">&apos;b&apos;</span>, <span class="hljs-string">&apos;c&apos;</span>].length <span class="hljs-comment">// 3</span>
</code></pre>
<p>JavaScript &#x4F7F;&#x7528;&#x4E00;&#x4E2A;32&#x4F4D;&#x6574;&#x6570;&#xFF0C;&#x4FDD;&#x5B58;&#x6570;&#x7EC4;&#x7684;&#x5143;&#x7D20;&#x4E2A;&#x6570;&#x3002;&#x8FD9;&#x610F;&#x5473;&#x7740;&#xFF0C;&#x6570;&#x7EC4;&#x6210;&#x5458;&#x6700;&#x591A;&#x53EA;&#x6709; 4294967295 &#x4E2A;&#xFF08;2<sup>32</sup> - 1&#xFF09;&#x4E2A;&#xFF0C;&#x4E5F;&#x5C31;&#x662F;&#x8BF4;<code>length</code>&#x5C5E;&#x6027;&#x7684;&#x6700;&#x5927;&#x503C;&#x5C31;&#x662F; 4294967295&#x3002;</p>
<p>&#x53EA;&#x8981;&#x662F;&#x6570;&#x7EC4;&#xFF0C;&#x5C31;&#x4E00;&#x5B9A;&#x6709;<code>length</code>&#x5C5E;&#x6027;&#x3002;&#x8BE5;&#x5C5E;&#x6027;&#x662F;&#x4E00;&#x4E2A;&#x52A8;&#x6001;&#x7684;&#x503C;&#xFF0C;&#x7B49;&#x4E8E;&#x952E;&#x540D;&#x4E2D;&#x7684;&#x6700;&#x5927;&#x6574;&#x6570;&#x52A0;&#x4E0A;<code>1</code>&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [<span class="hljs-string">&apos;a&apos;</span>, <span class="hljs-string">&apos;b&apos;</span>];
arr.length <span class="hljs-comment">// 2</span>

arr[<span class="hljs-number">2</span>] = <span class="hljs-string">&apos;c&apos;</span>;
arr.length <span class="hljs-comment">// 3</span>

arr[<span class="hljs-number">9</span>] = <span class="hljs-string">&apos;d&apos;</span>;
arr.length <span class="hljs-comment">// 10</span>

arr[<span class="hljs-number">1000</span>] = <span class="hljs-string">&apos;e&apos;</span>;
arr.length <span class="hljs-comment">// 1001</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x8868;&#x793A;&#xFF0C;&#x6570;&#x7EC4;&#x7684;&#x6570;&#x5B57;&#x952E;&#x4E0D;&#x9700;&#x8981;&#x8FDE;&#x7EED;&#xFF0C;<code>length</code>&#x5C5E;&#x6027;&#x7684;&#x503C;&#x603B;&#x662F;&#x6BD4;&#x6700;&#x5927;&#x7684;&#x90A3;&#x4E2A;&#x6574;&#x6570;&#x952E;&#x5927;<code>1</code>&#x3002;&#x53E6;&#x5916;&#xFF0C;&#x8FD9;&#x4E5F;&#x8868;&#x660E;&#x6570;&#x7EC4;&#x662F;&#x4E00;&#x79CD;&#x52A8;&#x6001;&#x7684;&#x6570;&#x636E;&#x7ED3;&#x6784;&#xFF0C;&#x53EF;&#x4EE5;&#x968F;&#x65F6;&#x589E;&#x51CF;&#x6570;&#x7EC4;&#x7684;&#x6210;&#x5458;&#x3002;</p>
<p><code>length</code>&#x5C5E;&#x6027;&#x662F;&#x53EF;&#x5199;&#x7684;&#x3002;&#x5982;&#x679C;&#x4EBA;&#x4E3A;&#x8BBE;&#x7F6E;&#x4E00;&#x4E2A;&#x5C0F;&#x4E8E;&#x5F53;&#x524D;&#x6210;&#x5458;&#x4E2A;&#x6570;&#x7684;&#x503C;&#xFF0C;&#x8BE5;&#x6570;&#x7EC4;&#x7684;&#x6210;&#x5458;&#x4F1A;&#x81EA;&#x52A8;&#x51CF;&#x5C11;&#x5230;<code>length</code>&#x8BBE;&#x7F6E;&#x7684;&#x503C;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [ <span class="hljs-string">&apos;a&apos;</span>, <span class="hljs-string">&apos;b&apos;</span>, <span class="hljs-string">&apos;c&apos;</span> ];
arr.length <span class="hljs-comment">// 3</span>

arr.length = <span class="hljs-number">2</span>;
arr <span class="hljs-comment">// [&quot;a&quot;, &quot;b&quot;]</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x8868;&#x793A;&#xFF0C;&#x5F53;&#x6570;&#x7EC4;&#x7684;<code>length</code>&#x5C5E;&#x6027;&#x8BBE;&#x4E3A;2&#xFF08;&#x5373;&#x6700;&#x5927;&#x7684;&#x6574;&#x6570;&#x952E;&#x53EA;&#x80FD;&#x662F;1&#xFF09;&#x90A3;&#x4E48;&#x6574;&#x6570;&#x952E;2&#xFF08;&#x503C;&#x4E3A;<code>c</code>&#xFF09;&#x5C31;&#x5DF2;&#x7ECF;&#x4E0D;&#x5728;&#x6570;&#x7EC4;&#x4E2D;&#x4E86;&#xFF0C;&#x88AB;&#x81EA;&#x52A8;&#x5220;&#x9664;&#x4E86;&#x3002;</p>
<p>&#x6E05;&#x7A7A;&#x6570;&#x7EC4;&#x7684;&#x4E00;&#x4E2A;&#x6709;&#x6548;&#x65B9;&#x6CD5;&#xFF0C;&#x5C31;&#x662F;&#x5C06;<code>length</code>&#x5C5E;&#x6027;&#x8BBE;&#x4E3A;0&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [ <span class="hljs-string">&apos;a&apos;</span>, <span class="hljs-string">&apos;b&apos;</span>, <span class="hljs-string">&apos;c&apos;</span> ];

arr.length = <span class="hljs-number">0</span>;
arr <span class="hljs-comment">// []</span>
</code></pre>
<p>&#x5982;&#x679C;&#x4EBA;&#x4E3A;&#x8BBE;&#x7F6E;<code>length</code>&#x5927;&#x4E8E;&#x5F53;&#x524D;&#x5143;&#x7D20;&#x4E2A;&#x6570;&#xFF0C;&#x5219;&#x6570;&#x7EC4;&#x7684;&#x6210;&#x5458;&#x6570;&#x91CF;&#x4F1A;&#x589E;&#x52A0;&#x5230;&#x8FD9;&#x4E2A;&#x503C;&#xFF0C;&#x65B0;&#x589E;&#x7684;&#x4F4D;&#x7F6E;&#x90FD;&#x662F;&#x7A7A;&#x4F4D;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [<span class="hljs-string">&apos;a&apos;</span>];

a.length = <span class="hljs-number">3</span>;
a[<span class="hljs-number">1</span>] <span class="hljs-comment">// undefined</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x8868;&#x793A;&#xFF0C;&#x5F53;<code>length</code>&#x5C5E;&#x6027;&#x8BBE;&#x4E3A;&#x5927;&#x4E8E;&#x6570;&#x7EC4;&#x4E2A;&#x6570;&#x65F6;&#xFF0C;&#x8BFB;&#x53D6;&#x65B0;&#x589E;&#x7684;&#x4F4D;&#x7F6E;&#x90FD;&#x4F1A;&#x8FD4;&#x56DE;<code>undefined</code>&#x3002;</p>
<p>&#x5982;&#x679C;&#x4EBA;&#x4E3A;&#x8BBE;&#x7F6E;<code>length</code>&#x4E3A;&#x4E0D;&#x5408;&#x6CD5;&#x7684;&#x503C;&#xFF0C;JavaScript &#x4F1A;&#x62A5;&#x9519;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-comment">// &#x8BBE;&#x7F6E;&#x8D1F;&#x503C;</span>
[].length = <span class="hljs-number">-1</span>
<span class="hljs-comment">// RangeError: Invalid array length</span>

<span class="hljs-comment">// &#x6570;&#x7EC4;&#x5143;&#x7D20;&#x4E2A;&#x6570;&#x5927;&#x4E8E;&#x7B49;&#x4E8E;2&#x7684;32&#x6B21;&#x65B9;</span>
[].length = <span class="hljs-built_in">Math</span>.pow(<span class="hljs-number">2</span>, <span class="hljs-number">32</span>)
<span class="hljs-comment">// RangeError: Invalid array length</span>

<span class="hljs-comment">// &#x8BBE;&#x7F6E;&#x5B57;&#x7B26;&#x4E32;</span>
[].length = <span class="hljs-string">&apos;abc&apos;</span>
<span class="hljs-comment">// RangeError: Invalid array length</span>
</code></pre>
<p>&#x503C;&#x5F97;&#x6CE8;&#x610F;&#x7684;&#x662F;&#xFF0C;&#x7531;&#x4E8E;&#x6570;&#x7EC4;&#x672C;&#x8D28;&#x4E0A;&#x662F;&#x4E00;&#x79CD;&#x5BF9;&#x8C61;&#xFF0C;&#x6240;&#x4EE5;&#x53EF;&#x4EE5;&#x4E3A;&#x6570;&#x7EC4;&#x6DFB;&#x52A0;&#x5C5E;&#x6027;&#xFF0C;&#x4F46;&#x662F;&#x8FD9;&#x4E0D;&#x5F71;&#x54CD;<code>length</code>&#x5C5E;&#x6027;&#x7684;&#x503C;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [];

a[<span class="hljs-string">&apos;p&apos;</span>] = <span class="hljs-string">&apos;abc&apos;</span>;
a.length <span class="hljs-comment">// 0</span>

a[<span class="hljs-number">2.1</span>] = <span class="hljs-string">&apos;abc&apos;</span>;
a.length <span class="hljs-comment">// 0</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x5C06;&#x6570;&#x7EC4;&#x7684;&#x952E;&#x5206;&#x522B;&#x8BBE;&#x4E3A;&#x5B57;&#x7B26;&#x4E32;&#x548C;&#x5C0F;&#x6570;&#xFF0C;&#x7ED3;&#x679C;&#x90FD;&#x4E0D;&#x5F71;&#x54CD;<code>length</code>&#x5C5E;&#x6027;&#x3002;&#x56E0;&#x4E3A;&#xFF0C;<code>length</code>&#x5C5E;&#x6027;&#x7684;&#x503C;&#x5C31;&#x662F;&#x7B49;&#x4E8E;&#x6700;&#x5927;&#x7684;&#x6570;&#x5B57;&#x952E;&#x52A0;1&#xFF0C;&#x800C;&#x8FD9;&#x4E2A;&#x6570;&#x7EC4;&#x6CA1;&#x6709;&#x6574;&#x6570;&#x952E;&#xFF0C;&#x6240;&#x4EE5;<code>length</code>&#x5C5E;&#x6027;&#x4FDD;&#x6301;&#x4E3A;<code>0</code>&#x3002;</p>
<p>&#x5982;&#x679C;&#x6570;&#x7EC4;&#x7684;&#x952E;&#x540D;&#x662F;&#x6DFB;&#x52A0;&#x8D85;&#x51FA;&#x8303;&#x56F4;&#x7684;&#x6570;&#x503C;&#xFF0C;&#x8BE5;&#x952E;&#x540D;&#x4F1A;&#x81EA;&#x52A8;&#x8F6C;&#x4E3A;&#x5B57;&#x7B26;&#x4E32;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [];
arr[<span class="hljs-number">-1</span>] = <span class="hljs-string">&apos;a&apos;</span>;
arr[<span class="hljs-built_in">Math</span>.pow(<span class="hljs-number">2</span>, <span class="hljs-number">32</span>)] = <span class="hljs-string">&apos;b&apos;</span>;

arr.length <span class="hljs-comment">// 0</span>
arr[<span class="hljs-number">-1</span>] <span class="hljs-comment">// &quot;a&quot;</span>
arr[<span class="hljs-number">4294967296</span>] <span class="hljs-comment">// &quot;b&quot;</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x4E2D;&#xFF0C;&#x6211;&#x4EEC;&#x4E3A;&#x6570;&#x7EC4;<code>arr</code>&#x6DFB;&#x52A0;&#x4E86;&#x4E24;&#x4E2A;&#x4E0D;&#x5408;&#x6CD5;&#x7684;&#x6570;&#x5B57;&#x952E;&#xFF0C;&#x7ED3;&#x679C;<code>length</code>&#x5C5E;&#x6027;&#x6CA1;&#x6709;&#x53D1;&#x751F;&#x53D8;&#x5316;&#x3002;&#x8FD9;&#x4E9B;&#x6570;&#x5B57;&#x952E;&#x90FD;&#x53D8;&#x6210;&#x4E86;&#x5B57;&#x7B26;&#x4E32;&#x952E;&#x540D;&#x3002;&#x6700;&#x540E;&#x4E24;&#x884C;&#x4E4B;&#x6240;&#x4EE5;&#x4F1A;&#x53D6;&#x5230;&#x503C;&#xFF0C;&#x662F;&#x56E0;&#x4E3A;&#x53D6;&#x952E;&#x503C;&#x65F6;&#xFF0C;&#x6570;&#x5B57;&#x952E;&#x540D;&#x4F1A;&#x9ED8;&#x8BA4;&#x8F6C;&#x4E3A;&#x5B57;&#x7B26;&#x4E32;&#x3002;</p>
<h2 id="in-&#x8FD0;&#x7B97;&#x7B26;">in &#x8FD0;&#x7B97;&#x7B26;</h2>
<p>&#x68C0;&#x67E5;&#x67D0;&#x4E2A;&#x952E;&#x540D;&#x662F;&#x5426;&#x5B58;&#x5728;&#x7684;&#x8FD0;&#x7B97;&#x7B26;<code>in</code>&#xFF0C;&#x9002;&#x7528;&#x4E8E;&#x5BF9;&#x8C61;&#xFF0C;&#x4E5F;&#x9002;&#x7528;&#x4E8E;&#x6570;&#x7EC4;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [ <span class="hljs-string">&apos;a&apos;</span>, <span class="hljs-string">&apos;b&apos;</span>, <span class="hljs-string">&apos;c&apos;</span> ];
<span class="hljs-number">2</span> <span class="hljs-keyword">in</span> arr  <span class="hljs-comment">// true</span>
<span class="hljs-string">&apos;2&apos;</span> <span class="hljs-keyword">in</span> arr <span class="hljs-comment">// true</span>
<span class="hljs-number">4</span> <span class="hljs-keyword">in</span> arr <span class="hljs-comment">// false</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x8868;&#x660E;&#xFF0C;&#x6570;&#x7EC4;&#x5B58;&#x5728;&#x952E;&#x540D;&#x4E3A;<code>2</code>&#x7684;&#x952E;&#x3002;&#x7531;&#x4E8E;&#x952E;&#x540D;&#x90FD;&#x662F;&#x5B57;&#x7B26;&#x4E32;&#xFF0C;&#x6240;&#x4EE5;&#x6570;&#x503C;<code>2</code>&#x4F1A;&#x81EA;&#x52A8;&#x8F6C;&#x6210;&#x5B57;&#x7B26;&#x4E32;&#x3002;</p>
<p>&#x6CE8;&#x610F;&#xFF0C;&#x5982;&#x679C;&#x6570;&#x7EC4;&#x7684;&#x67D0;&#x4E2A;&#x4F4D;&#x7F6E;&#x662F;&#x7A7A;&#x4F4D;&#xFF0C;<code>in</code>&#x8FD0;&#x7B97;&#x7B26;&#x8FD4;&#x56DE;<code>false</code>&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = [];
arr[<span class="hljs-number">100</span>] = <span class="hljs-string">&apos;a&apos;</span>;

<span class="hljs-number">100</span> <span class="hljs-keyword">in</span> arr <span class="hljs-comment">// true</span>
<span class="hljs-number">1</span> <span class="hljs-keyword">in</span> arr <span class="hljs-comment">// false</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x4E2D;&#xFF0C;&#x6570;&#x7EC4;<code>arr</code>&#x53EA;&#x6709;&#x4E00;&#x4E2A;&#x6210;&#x5458;<code>arr[100]</code>&#xFF0C;&#x5176;&#x4ED6;&#x4F4D;&#x7F6E;&#x7684;&#x952E;&#x540D;&#x90FD;&#x4F1A;&#x8FD4;&#x56DE;<code>false</code>&#x3002;</p>
<h2 id="forin-&#x5FAA;&#x73AF;&#x548C;&#x6570;&#x7EC4;&#x7684;&#x904D;&#x5386;">for...in &#x5FAA;&#x73AF;&#x548C;&#x6570;&#x7EC4;&#x7684;&#x904D;&#x5386;</h2>
<p><code>for...in</code>&#x5FAA;&#x73AF;&#x4E0D;&#x4EC5;&#x53EF;&#x4EE5;&#x904D;&#x5386;&#x5BF9;&#x8C61;&#xFF0C;&#x4E5F;&#x53EF;&#x4EE5;&#x904D;&#x5386;&#x6570;&#x7EC4;&#xFF0C;&#x6BD5;&#x7ADF;&#x6570;&#x7EC4;&#x53EA;&#x662F;&#x4E00;&#x79CD;&#x7279;&#x6B8A;&#x5BF9;&#x8C61;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>];

<span class="hljs-keyword">for</span> (<span class="hljs-keyword">var</span> i <span class="hljs-keyword">in</span> a) {
  <span class="hljs-built_in">console</span>.log(a[i]);
}
<span class="hljs-comment">// 1</span>
<span class="hljs-comment">// 2</span>
<span class="hljs-comment">// 3</span>
</code></pre>
<p>&#x4F46;&#x662F;&#xFF0C;<code>for...in</code>&#x4E0D;&#x4EC5;&#x4F1A;&#x904D;&#x5386;&#x6570;&#x7EC4;&#x6240;&#x6709;&#x7684;&#x6570;&#x5B57;&#x952E;&#xFF0C;&#x8FD8;&#x4F1A;&#x904D;&#x5386;&#x975E;&#x6570;&#x5B57;&#x952E;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>];
a.foo = <span class="hljs-literal">true</span>;

<span class="hljs-keyword">for</span> (<span class="hljs-keyword">var</span> key <span class="hljs-keyword">in</span> a) {
  <span class="hljs-built_in">console</span>.log(key);
}
<span class="hljs-comment">// 0</span>
<span class="hljs-comment">// 1</span>
<span class="hljs-comment">// 2</span>
<span class="hljs-comment">// foo</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x5728;&#x904D;&#x5386;&#x6570;&#x7EC4;&#x65F6;&#xFF0C;&#x4E5F;&#x904D;&#x5386;&#x5230;&#x4E86;&#x975E;&#x6574;&#x6570;&#x952E;<code>foo</code>&#x3002;&#x6240;&#x4EE5;&#xFF0C;&#x4E0D;&#x63A8;&#x8350;&#x4F7F;&#x7528;<code>for...in</code>&#x904D;&#x5386;&#x6570;&#x7EC4;&#x3002;</p>
<p>&#x6570;&#x7EC4;&#x7684;&#x904D;&#x5386;&#x53EF;&#x4EE5;&#x8003;&#x8651;&#x4F7F;&#x7528;<code>for</code>&#x5FAA;&#x73AF;&#x6216;<code>while</code>&#x5FAA;&#x73AF;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>];

<span class="hljs-comment">// for&#x5FAA;&#x73AF;</span>
<span class="hljs-keyword">for</span>(<span class="hljs-keyword">var</span> i = <span class="hljs-number">0</span>; i &lt; a.length; i++) {
  <span class="hljs-built_in">console</span>.log(a[i]);
}

<span class="hljs-comment">// while&#x5FAA;&#x73AF;</span>
<span class="hljs-keyword">var</span> i = <span class="hljs-number">0</span>;
<span class="hljs-keyword">while</span> (i &lt; a.length) {
  <span class="hljs-built_in">console</span>.log(a[i]);
  i++;
}

<span class="hljs-keyword">var</span> l = a.length;
<span class="hljs-keyword">while</span> (l--) {
  <span class="hljs-built_in">console</span>.log(a[l]);
}
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x662F;&#x4E09;&#x79CD;&#x904D;&#x5386;&#x6570;&#x7EC4;&#x7684;&#x5199;&#x6CD5;&#x3002;&#x6700;&#x540E;&#x4E00;&#x79CD;&#x5199;&#x6CD5;&#x662F;&#x9006;&#x5411;&#x904D;&#x5386;&#xFF0C;&#x5373;&#x4ECE;&#x6700;&#x540E;&#x4E00;&#x4E2A;&#x5143;&#x7D20;&#x5411;&#x7B2C;&#x4E00;&#x4E2A;&#x5143;&#x7D20;&#x904D;&#x5386;&#x3002;</p>
<p>&#x6570;&#x7EC4;&#x7684;<code>forEach</code>&#x65B9;&#x6CD5;&#xFF0C;&#x4E5F;&#x53EF;&#x4EE5;&#x7528;&#x6765;&#x904D;&#x5386;&#x6570;&#x7EC4;&#xFF0C;&#x8BE6;&#x89C1;&#x300A;&#x6807;&#x51C6;&#x5E93;&#x300B;&#x7684; Array &#x5BF9;&#x8C61;&#x4E00;&#x7AE0;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> colors = [<span class="hljs-string">&apos;red&apos;</span>, <span class="hljs-string">&apos;green&apos;</span>, <span class="hljs-string">&apos;blue&apos;</span>];
colors.forEach(<span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params">color</span>) </span>{
  <span class="hljs-built_in">console</span>.log(color);
});
<span class="hljs-comment">// red</span>
<span class="hljs-comment">// green</span>
<span class="hljs-comment">// blue</span>
</code></pre>
<h2 id="&#x6570;&#x7EC4;&#x7684;&#x7A7A;&#x4F4D;">&#x6570;&#x7EC4;&#x7684;&#x7A7A;&#x4F4D;</h2>
<p>&#x5F53;&#x6570;&#x7EC4;&#x7684;&#x67D0;&#x4E2A;&#x4F4D;&#x7F6E;&#x662F;&#x7A7A;&#x5143;&#x7D20;&#xFF0C;&#x5373;&#x4E24;&#x4E2A;&#x9017;&#x53F7;&#x4E4B;&#x95F4;&#x6CA1;&#x6709;&#x4EFB;&#x4F55;&#x503C;&#xFF0C;&#x6211;&#x4EEC;&#x79F0;&#x8BE5;&#x6570;&#x7EC4;&#x5B58;&#x5728;&#x7A7A;&#x4F4D;&#xFF08;hole&#xFF09;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [<span class="hljs-number">1</span>, , <span class="hljs-number">1</span>];
a.length <span class="hljs-comment">// 3</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x8868;&#x660E;&#xFF0C;&#x6570;&#x7EC4;&#x7684;&#x7A7A;&#x4F4D;&#x4E0D;&#x5F71;&#x54CD;<code>length</code>&#x5C5E;&#x6027;&#x3002;</p>
<p>&#x9700;&#x8981;&#x6CE8;&#x610F;&#x7684;&#x662F;&#xFF0C;&#x5982;&#x679C;&#x6700;&#x540E;&#x4E00;&#x4E2A;&#x5143;&#x7D20;&#x540E;&#x9762;&#x6709;&#x9017;&#x53F7;&#xFF0C;&#x5E76;&#x4E0D;&#x4F1A;&#x4EA7;&#x751F;&#x7A7A;&#x4F4D;&#x3002;&#x4E5F;&#x5C31;&#x662F;&#x8BF4;&#xFF0C;&#x6709;&#x6CA1;&#x6709;&#x8FD9;&#x4E2A;&#x9017;&#x53F7;&#xFF0C;&#x7ED3;&#x679C;&#x90FD;&#x662F;&#x4E00;&#x6837;&#x7684;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>,];

a.length <span class="hljs-comment">// 3</span>
a <span class="hljs-comment">// [1, 2, 3]</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x4E2D;&#xFF0C;&#x6570;&#x7EC4;&#x6700;&#x540E;&#x4E00;&#x4E2A;&#x6210;&#x5458;&#x540E;&#x9762;&#x6709;&#x4E00;&#x4E2A;&#x9017;&#x53F7;&#xFF0C;&#x8FD9;&#x4E0D;&#x5F71;&#x54CD;<code>length</code>&#x5C5E;&#x6027;&#x7684;&#x503C;&#xFF0C;&#x4E0E;&#x6CA1;&#x6709;&#x8FD9;&#x4E2A;&#x9017;&#x53F7;&#x65F6;&#x6548;&#x679C;&#x4E00;&#x6837;&#x3002;</p>
<p>&#x6570;&#x7EC4;&#x7684;&#x7A7A;&#x4F4D;&#x662F;&#x53EF;&#x4EE5;&#x8BFB;&#x53D6;&#x7684;&#xFF0C;&#x8FD4;&#x56DE;<code>undefined</code>&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [, , ,];
a[<span class="hljs-number">1</span>] <span class="hljs-comment">// undefined</span>
</code></pre>
<p>&#x4F7F;&#x7528;<code>delete</code>&#x547D;&#x4EE4;&#x5220;&#x9664;&#x4E00;&#x4E2A;&#x6570;&#x7EC4;&#x6210;&#x5458;&#xFF0C;&#x4F1A;&#x5F62;&#x6210;&#x7A7A;&#x4F4D;&#xFF0C;&#x5E76;&#x4E14;&#x4E0D;&#x4F1A;&#x5F71;&#x54CD;<code>length</code>&#x5C5E;&#x6027;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>];
<span class="hljs-keyword">delete</span> a[<span class="hljs-number">1</span>];

a[<span class="hljs-number">1</span>] <span class="hljs-comment">// undefined</span>
a.length <span class="hljs-comment">// 3</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x7528;<code>delete</code>&#x547D;&#x4EE4;&#x5220;&#x9664;&#x4E86;&#x6570;&#x7EC4;&#x7684;&#x7B2C;&#x4E8C;&#x4E2A;&#x5143;&#x7D20;&#xFF0C;&#x8FD9;&#x4E2A;&#x4F4D;&#x7F6E;&#x5C31;&#x5F62;&#x6210;&#x4E86;&#x7A7A;&#x4F4D;&#xFF0C;&#x4F46;&#x662F;&#x5BF9;<code>length</code>&#x5C5E;&#x6027;&#x6CA1;&#x6709;&#x5F71;&#x54CD;&#x3002;&#x4E5F;&#x5C31;&#x662F;&#x8BF4;&#xFF0C;<code>length</code>&#x5C5E;&#x6027;&#x4E0D;&#x8FC7;&#x6EE4;&#x7A7A;&#x4F4D;&#x3002;&#x6240;&#x4EE5;&#xFF0C;&#x4F7F;&#x7528;<code>length</code>&#x5C5E;&#x6027;&#x8FDB;&#x884C;&#x6570;&#x7EC4;&#x904D;&#x5386;&#xFF0C;&#x4E00;&#x5B9A;&#x8981;&#x975E;&#x5E38;&#x5C0F;&#x5FC3;&#x3002;</p>
<p>&#x6570;&#x7EC4;&#x7684;&#x67D0;&#x4E2A;&#x4F4D;&#x7F6E;&#x662F;&#x7A7A;&#x4F4D;&#xFF0C;&#x4E0E;&#x67D0;&#x4E2A;&#x4F4D;&#x7F6E;&#x662F;<code>undefined</code>&#xFF0C;&#x662F;&#x4E0D;&#x4E00;&#x6837;&#x7684;&#x3002;&#x5982;&#x679C;&#x662F;&#x7A7A;&#x4F4D;&#xFF0C;&#x4F7F;&#x7528;&#x6570;&#x7EC4;&#x7684;<code>forEach</code>&#x65B9;&#x6CD5;&#x3001;<code>for...in</code>&#x7ED3;&#x6784;&#x3001;&#x4EE5;&#x53CA;<code>Object.keys</code>&#x65B9;&#x6CD5;&#x8FDB;&#x884C;&#x904D;&#x5386;&#xFF0C;&#x7A7A;&#x4F4D;&#x90FD;&#x4F1A;&#x88AB;&#x8DF3;&#x8FC7;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [, , ,];

a.forEach(<span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params">x, i</span>) </span>{
  <span class="hljs-built_in">console</span>.log(i + <span class="hljs-string">&apos;. &apos;</span> + x);
})
<span class="hljs-comment">// &#x4E0D;&#x4EA7;&#x751F;&#x4EFB;&#x4F55;&#x8F93;&#x51FA;</span>

<span class="hljs-keyword">for</span> (<span class="hljs-keyword">var</span> i <span class="hljs-keyword">in</span> a) {
  <span class="hljs-built_in">console</span>.log(i);
}
<span class="hljs-comment">// &#x4E0D;&#x4EA7;&#x751F;&#x4EFB;&#x4F55;&#x8F93;&#x51FA;</span>

<span class="hljs-built_in">Object</span>.keys(a)
<span class="hljs-comment">// []</span>
</code></pre>
<p>&#x5982;&#x679C;&#x67D0;&#x4E2A;&#x4F4D;&#x7F6E;&#x662F;<code>undefined</code>&#xFF0C;&#x904D;&#x5386;&#x7684;&#x65F6;&#x5019;&#x5C31;&#x4E0D;&#x4F1A;&#x88AB;&#x8DF3;&#x8FC7;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> a = [<span class="hljs-literal">undefined</span>, <span class="hljs-literal">undefined</span>, <span class="hljs-literal">undefined</span>];

a.forEach(<span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params">x, i</span>) </span>{
  <span class="hljs-built_in">console</span>.log(i + <span class="hljs-string">&apos;. &apos;</span> + x);
});
<span class="hljs-comment">// 0. undefined</span>
<span class="hljs-comment">// 1. undefined</span>
<span class="hljs-comment">// 2. undefined</span>

<span class="hljs-keyword">for</span> (<span class="hljs-keyword">var</span> i <span class="hljs-keyword">in</span> a) {
  <span class="hljs-built_in">console</span>.log(i);
}
<span class="hljs-comment">// 0</span>
<span class="hljs-comment">// 1</span>
<span class="hljs-comment">// 2</span>

<span class="hljs-built_in">Object</span>.keys(a)
<span class="hljs-comment">// [&apos;0&apos;, &apos;1&apos;, &apos;2&apos;]</span>
</code></pre>
<p>&#x8FD9;&#x5C31;&#x662F;&#x8BF4;&#xFF0C;&#x7A7A;&#x4F4D;&#x5C31;&#x662F;&#x6570;&#x7EC4;&#x6CA1;&#x6709;&#x8FD9;&#x4E2A;&#x5143;&#x7D20;&#xFF0C;&#x6240;&#x4EE5;&#x4E0D;&#x4F1A;&#x88AB;&#x904D;&#x5386;&#x5230;&#xFF0C;&#x800C;<code>undefined</code>&#x5219;&#x8868;&#x793A;&#x6570;&#x7EC4;&#x6709;&#x8FD9;&#x4E2A;&#x5143;&#x7D20;&#xFF0C;&#x503C;&#x662F;<code>undefined</code>&#xFF0C;&#x6240;&#x4EE5;&#x904D;&#x5386;&#x4E0D;&#x4F1A;&#x8DF3;&#x8FC7;&#x3002;</p>
<h2 id="&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;">&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;</h2>
<p>&#x5982;&#x679C;&#x4E00;&#x4E2A;&#x5BF9;&#x8C61;&#x7684;&#x6240;&#x6709;&#x952E;&#x540D;&#x90FD;&#x662F;&#x6B63;&#x6574;&#x6570;&#x6216;&#x96F6;&#xFF0C;&#x5E76;&#x4E14;&#x6709;<code>length</code>&#x5C5E;&#x6027;&#xFF0C;&#x90A3;&#x4E48;&#x8FD9;&#x4E2A;&#x5BF9;&#x8C61;&#x5C31;&#x5F88;&#x50CF;&#x6570;&#x7EC4;&#xFF0C;&#x8BED;&#x6CD5;&#x4E0A;&#x79F0;&#x4E3A;&#x201C;&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;&#x201D;&#xFF08;array-like object&#xFF09;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> obj = {
  <span class="hljs-number">0</span>: <span class="hljs-string">&apos;a&apos;</span>,
  <span class="hljs-number">1</span>: <span class="hljs-string">&apos;b&apos;</span>,
  <span class="hljs-number">2</span>: <span class="hljs-string">&apos;c&apos;</span>,
  length: <span class="hljs-number">3</span>
};

obj[<span class="hljs-number">0</span>] <span class="hljs-comment">// &apos;a&apos;</span>
obj[<span class="hljs-number">1</span>] <span class="hljs-comment">// &apos;b&apos;</span>
obj.length <span class="hljs-comment">// 3</span>
obj.push(<span class="hljs-string">&apos;d&apos;</span>) <span class="hljs-comment">// TypeError: obj.push is not a function</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x4E2D;&#xFF0C;&#x5BF9;&#x8C61;<code>obj</code>&#x5C31;&#x662F;&#x4E00;&#x4E2A;&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;&#x3002;&#x4F46;&#x662F;&#xFF0C;&#x201C;&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;&#x201D;&#x5E76;&#x4E0D;&#x662F;&#x6570;&#x7EC4;&#xFF0C;&#x56E0;&#x4E3A;&#x5B83;&#x4EEC;&#x4E0D;&#x5177;&#x5907;&#x6570;&#x7EC4;&#x7279;&#x6709;&#x7684;&#x65B9;&#x6CD5;&#x3002;&#x5BF9;&#x8C61;<code>obj</code>&#x6CA1;&#x6709;&#x6570;&#x7EC4;&#x7684;<code>push</code>&#x65B9;&#x6CD5;&#xFF0C;&#x4F7F;&#x7528;&#x8BE5;&#x65B9;&#x6CD5;&#x5C31;&#x4F1A;&#x62A5;&#x9519;&#x3002;</p>
<p>&#x201C;&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;&#x201D;&#x7684;&#x6839;&#x672C;&#x7279;&#x5F81;&#xFF0C;&#x5C31;&#x662F;&#x5177;&#x6709;<code>length</code>&#x5C5E;&#x6027;&#x3002;&#x53EA;&#x8981;&#x6709;<code>length</code>&#x5C5E;&#x6027;&#xFF0C;&#x5C31;&#x53EF;&#x4EE5;&#x8BA4;&#x4E3A;&#x8FD9;&#x4E2A;&#x5BF9;&#x8C61;&#x7C7B;&#x4F3C;&#x4E8E;&#x6570;&#x7EC4;&#x3002;&#x4F46;&#x662F;&#x6709;&#x4E00;&#x4E2A;&#x95EE;&#x9898;&#xFF0C;&#x8FD9;&#x79CD;<code>length</code>&#x5C5E;&#x6027;&#x4E0D;&#x662F;&#x52A8;&#x6001;&#x503C;&#xFF0C;&#x4E0D;&#x4F1A;&#x968F;&#x7740;&#x6210;&#x5458;&#x7684;&#x53D8;&#x5316;&#x800C;&#x53D8;&#x5316;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> obj = {
  length: <span class="hljs-number">0</span>
};
obj[<span class="hljs-number">3</span>] = <span class="hljs-string">&apos;d&apos;</span>;
obj.length <span class="hljs-comment">// 0</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x4E3A;&#x5BF9;&#x8C61;<code>obj</code>&#x6DFB;&#x52A0;&#x4E86;&#x4E00;&#x4E2A;&#x6570;&#x5B57;&#x952E;&#xFF0C;&#x4F46;&#x662F;<code>length</code>&#x5C5E;&#x6027;&#x6CA1;&#x53D8;&#x3002;&#x8FD9;&#x5C31;&#x8BF4;&#x660E;&#x4E86;<code>obj</code>&#x4E0D;&#x662F;&#x6570;&#x7EC4;&#x3002;</p>
<p>&#x5178;&#x578B;&#x7684;&#x201C;&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;&#x201D;&#x662F;&#x51FD;&#x6570;&#x7684;<code>arguments</code>&#x5BF9;&#x8C61;&#xFF0C;&#x4EE5;&#x53CA;&#x5927;&#x591A;&#x6570; DOM &#x5143;&#x7D20;&#x96C6;&#xFF0C;&#x8FD8;&#x6709;&#x5B57;&#x7B26;&#x4E32;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-comment">// arguments&#x5BF9;&#x8C61;</span>
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">args</span>(<span class="hljs-params"></span>) </span>{ <span class="hljs-keyword">return</span> <span class="hljs-built_in">arguments</span> }
<span class="hljs-keyword">var</span> arrayLike = args(<span class="hljs-string">&apos;a&apos;</span>, <span class="hljs-string">&apos;b&apos;</span>);

arrayLike[<span class="hljs-number">0</span>] <span class="hljs-comment">// &apos;a&apos;</span>
arrayLike.length <span class="hljs-comment">// 2</span>
arrayLike <span class="hljs-keyword">instanceof</span> <span class="hljs-built_in">Array</span> <span class="hljs-comment">// false</span>

<span class="hljs-comment">// DOM&#x5143;&#x7D20;&#x96C6;</span>
<span class="hljs-keyword">var</span> elts = <span class="hljs-built_in">document</span>.getElementsByTagName(<span class="hljs-string">&apos;h3&apos;</span>);
elts.length <span class="hljs-comment">// 3</span>
elts <span class="hljs-keyword">instanceof</span> <span class="hljs-built_in">Array</span> <span class="hljs-comment">// false</span>

<span class="hljs-comment">// &#x5B57;&#x7B26;&#x4E32;</span>
<span class="hljs-string">&apos;abc&apos;</span>[<span class="hljs-number">1</span>] <span class="hljs-comment">// &apos;b&apos;</span>
<span class="hljs-string">&apos;abc&apos;</span>.length <span class="hljs-comment">// 3</span>
<span class="hljs-string">&apos;abc&apos;</span> <span class="hljs-keyword">instanceof</span> <span class="hljs-built_in">Array</span> <span class="hljs-comment">// false</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x5305;&#x542B;&#x4E09;&#x4E2A;&#x4F8B;&#x5B50;&#xFF0C;&#x5B83;&#x4EEC;&#x90FD;&#x4E0D;&#x662F;&#x6570;&#x7EC4;&#xFF08;<code>instanceof</code>&#x8FD0;&#x7B97;&#x7B26;&#x8FD4;&#x56DE;<code>false</code>&#xFF09;&#xFF0C;&#x4F46;&#x662F;&#x770B;&#x4E0A;&#x53BB;&#x90FD;&#x975E;&#x5E38;&#x50CF;&#x6570;&#x7EC4;&#x3002;</p>
<p>&#x6570;&#x7EC4;&#x7684;<code>slice</code>&#x65B9;&#x6CD5;&#x53EF;&#x4EE5;&#x5C06;&#x201C;&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;&#x201D;&#x53D8;&#x6210;&#x771F;&#x6B63;&#x7684;&#x6570;&#x7EC4;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = <span class="hljs-built_in">Array</span>.prototype.slice.call(arrayLike);
</code></pre>
<p>&#x9664;&#x4E86;&#x8F6C;&#x4E3A;&#x771F;&#x6B63;&#x7684;&#x6570;&#x7EC4;&#xFF0C;&#x201C;&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;&#x201D;&#x8FD8;&#x6709;&#x4E00;&#x4E2A;&#x529E;&#x6CD5;&#x53EF;&#x4EE5;&#x4F7F;&#x7528;&#x6570;&#x7EC4;&#x7684;&#x65B9;&#x6CD5;&#xFF0C;&#x5C31;&#x662F;&#x901A;&#x8FC7;<code>call()</code>&#x628A;&#x6570;&#x7EC4;&#x7684;&#x65B9;&#x6CD5;&#x653E;&#x5230;&#x5BF9;&#x8C61;&#x4E0A;&#x9762;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">print</span>(<span class="hljs-params">value, index</span>) </span>{
  <span class="hljs-built_in">console</span>.log(index + <span class="hljs-string">&apos; : &apos;</span> + value);
}

<span class="hljs-built_in">Array</span>.prototype.forEach.call(arrayLike, print);
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x4E2D;&#xFF0C;<code>arrayLike</code>&#x4EE3;&#x8868;&#x4E00;&#x4E2A;&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;&#xFF0C;&#x672C;&#x6765;&#x662F;&#x4E0D;&#x53EF;&#x4EE5;&#x4F7F;&#x7528;&#x6570;&#x7EC4;&#x7684;<code>forEach()</code>&#x65B9;&#x6CD5;&#x7684;&#xFF0C;&#x4F46;&#x662F;&#x901A;&#x8FC7;<code>call()</code>&#xFF0C;&#x53EF;&#x4EE5;&#x628A;<code>forEach()</code>&#x5AC1;&#x63A5;&#x5230;<code>arrayLike</code>&#x4E0A;&#x9762;&#x8C03;&#x7528;&#x3002;</p>
<p>&#x4E0B;&#x9762;&#x7684;&#x4F8B;&#x5B50;&#x5C31;&#x662F;&#x901A;&#x8FC7;&#x8FD9;&#x79CD;&#x65B9;&#x6CD5;&#xFF0C;&#x5728;<code>arguments</code>&#x5BF9;&#x8C61;&#x4E0A;&#x9762;&#x8C03;&#x7528;<code>forEach</code>&#x65B9;&#x6CD5;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-comment">// forEach &#x65B9;&#x6CD5;</span>
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">logArgs</span>(<span class="hljs-params"></span>) </span>{
  <span class="hljs-built_in">Array</span>.prototype.forEach.call(<span class="hljs-built_in">arguments</span>, <span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params">elem, i</span>) </span>{
    <span class="hljs-built_in">console</span>.log(i + <span class="hljs-string">&apos;. &apos;</span> + elem);
  });
}

<span class="hljs-comment">// &#x7B49;&#x540C;&#x4E8E; for &#x5FAA;&#x73AF;</span>
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">logArgs</span>(<span class="hljs-params"></span>) </span>{
  <span class="hljs-keyword">for</span> (<span class="hljs-keyword">var</span> i = <span class="hljs-number">0</span>; i &lt; <span class="hljs-built_in">arguments</span>.length; i++) {
    <span class="hljs-built_in">console</span>.log(i + <span class="hljs-string">&apos;. &apos;</span> + <span class="hljs-built_in">arguments</span>[i]);
  }
}
</code></pre>
<p>&#x5B57;&#x7B26;&#x4E32;&#x4E5F;&#x662F;&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;&#xFF0C;&#x6240;&#x4EE5;&#x4E5F;&#x53EF;&#x4EE5;&#x7528;<code>Array.prototype.forEach.call</code>&#x904D;&#x5386;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-built_in">Array</span>.prototype.forEach.call(<span class="hljs-string">&apos;abc&apos;</span>, <span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params">chr</span>) </span>{
  <span class="hljs-built_in">console</span>.log(chr);
});
<span class="hljs-comment">// a</span>
<span class="hljs-comment">// b</span>
<span class="hljs-comment">// c</span>
</code></pre>
<p>&#x6CE8;&#x610F;&#xFF0C;&#x8FD9;&#x79CD;&#x65B9;&#x6CD5;&#x6BD4;&#x76F4;&#x63A5;&#x4F7F;&#x7528;&#x6570;&#x7EC4;&#x539F;&#x751F;&#x7684;<code>forEach</code>&#x8981;&#x6162;&#xFF0C;&#x6240;&#x4EE5;&#x6700;&#x597D;&#x8FD8;&#x662F;&#x5148;&#x5C06;&#x201C;&#x7C7B;&#x4F3C;&#x6570;&#x7EC4;&#x7684;&#x5BF9;&#x8C61;&#x201D;&#x8F6C;&#x4E3A;&#x771F;&#x6B63;&#x7684;&#x6570;&#x7EC4;&#xFF0C;&#x7136;&#x540E;&#x518D;&#x76F4;&#x63A5;&#x8C03;&#x7528;&#x6570;&#x7EC4;&#x7684;<code>forEach</code>&#x65B9;&#x6CD5;&#x3002;</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">var</span> arr = <span class="hljs-built_in">Array</span>.prototype.slice.call(<span class="hljs-string">&apos;abc&apos;</span>);
arr.forEach(<span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params">chr</span>) </span>{
  <span class="hljs-built_in">console</span>.log(chr);
});
<span class="hljs-comment">// a</span>
<span class="hljs-comment">// b</span>
<span class="hljs-comment">// c</span>
</code></pre>
<h2 id="&#x53C2;&#x8003;&#x94FE;&#x63A5;">&#x53C2;&#x8003;&#x94FE;&#x63A5;</h2>
<ul>
<li>Axel Rauschmayer, <a href="http://www.2ality.com/2012/12/arrays.html" target="_blank" rel="nofollow">Arrays in JavaScript</a></li>
<li>Axel Rauschmayer, <a href="http://www.2ality.com/2012/06/dense-arrays.html" target="_blank" rel="nofollow">JavaScript: sparse arrays vs. dense arrays</a></li>
<li>Felix Bohm, <a href="http://net.tutsplus.com/tutorials/javascript-ajax/what-they-didnt-tell-you-about-es5s-array-extras/" target="_blank" rel="nofollow">What They Didn&#x2019;t Tell You About ES5&#x2032;s Array Extras</a></li>
<li>Juriy Zaytsev, <a href="http://perfectionkills.com/how-ecmascript-5-still-does-not-allow-to-subclass-an-array/" target="_blank" rel="nofollow">How ECMAScript 5 still does not allow to subclass an array</a></li>
</ul>




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